1. Technical Field
The present invention relates to an inkjet printing machine configured to perform printing by ejecting inks onto a sheet from ink heads.
2. Related Art
There have been line-type inkjet printing machines configured to eject an ink onto a sheet from a fixed ink head while transferring the sheet. Such a line-type inkjet printing machine uses an ink head formed of multiple head modules aligned in the print width direction (main scanning direction) (see Japanese Patent Application Publication No. 2012-66510).
In a line-type inkjet printing machine capable of color printing with inks of multiple colors, multiple ink heads configured to eject inks of different colors are arranged at certain intervals in the sheet transfer direction (sub scanning direction). For color printing, the ink heads are driven at different timings so that the inks of the different colors ejected from the nozzles of the ink heads can land and overlap each other in each pixel on the transferred sheet.
The head modules forming the ink heads are attached to a head holder. It is difficult to precisely adjust the positions of the head modules so that the inks of the different colors can land on their respective right positions without shifting in each pixel because a large amount of man-hours and high cost are required for the attachment. For this reason, the head modules are often offset from the right positions.
The offset in the attached positions of the head modules in the sub scanning direction has heretofore been compensated by adjusting the ink ejection timings of the head modules. On the other hand, as for the main scanning direction, which nozzles to use for each pixel of image data have been chosen for each head module line so as to minimize the offset in the positions, in the main scanning direction, of the nozzles of the head modules of the different colors covering the same pixel. Here, each head module line is formed of head modules of the different colors arranged in a line along the sub scanning direction.